1. Field of the Invention
The present invention relates to a method of formatting a storage medium into sectors for storing information. The invention also relates to a drive unit that can store data in and retrieve data from such a storage medium.
2. Description of the Prior Art
In recent years, storage media used as external storage for electronic computers have tended to have higher storage densities. With this tendency the amount of errors contained in the information read from a storage medium having defects has tended to increase. Accordingly, it is the common practice to append error-correcting codes (ECCs) to the information to be written. However, a considerable time is taken to decode encoded words containing such error-correcting codes. Therefore, these error-correcting codes which provide powerful error correction are not generally added to each ID (identification) field storing ID information including addresses, but rather only error-detecting codes, such as cyclic redundancy codes (CRCs) are appended to ID fields, because a sufficient time cannot be obtained during the period which begins with the reading of data from one ID field and ends with the arrival at the subsequent data field. Accordingly, a formatting has been proposed in which an ID block including cyclic redundancy codes as well as ID information about the identification of the sector is repeatedly written at consecutive locations within the ID field of each sector.
It is possible to enhance the reliability with which ID information is read out, by this conventional method of formatting. Unfortunately, erroneous reading of information tends to take place consecutively through several bytes, which is generally known as a burst error. If this burst error is especially long, it would occur that any of the same ID information written at some locations cannot be read. In this way, a sufficient reliability cannot be offered by merely writing the same ID information at a plurality of locations on the storage medium that tends to produce long burst errors. Also, it is necessary that the same ID block be written at more locations in order to provide immunity to long burst errors. In this case, there arises the problem that the redundancy increases.